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Sept. 26, 2011 -- Some coffee drinkers may have a reason to smile -- or keep smiling.

Drinking several cups a day is linked with a lower risk of depression, according to a new study that looked only at women. The benefit seems to start at two cups a day.

The study is published in the Archives of Internal Medicine .

"We found that women with the highest coffee intake -- those in the top 20% -- had a 20% lower risk of developing depression," says researcher Alberto Ascherio, MD, DrPH. "The top 20% drank about the equivalent of four cups of coffee per day." Ascherio is professor of epidemiology and nutrition at the Harvard School of Public Health and professor of medicine at Harvard Medical School.

The link was not found for decaffeinated coffee.

Ascherio emphasizes that he found an association, not cause and effect. Depression affects twice as many women as men, he notes. About 20% of U.S. women will be affected during their lifetime.

A mental health expert cautions that the research finding should be viewed cautiously. "I wouldn't tell my patients to drink more caffeine based on this study," says Michelle Riba, MD, associate director of the University of Michigan Depression Center. She reviewed the study but was not involved in it.

Caffeine and Depression Risk

Ascherio and colleagues evaluated more than 50,000 women, average age 63. They were participating in the long-running Nurses' Health Study.

None had any depression at the start of the investigation in 1996. All answered questions about their caffeine habits through June 2006. "Women reported their consumption of coffee and other caffeine drinks, more or less every four years," Ascherio says. They noted their usual intake over the past 12 months.

The women also reported if they had clinical depression or if they began using antidepressant medication.

During the follow-up, 2,607 women developed depression.

Overall, the more coffee they drank, the less likely they were to become depressed. "We see a dose response," Ascherio tells WebMD.

Compared with women who drank one cup or less a week, those who had two to three cups a day had a 15% lower risk of depression. Those who drank four or more had a 20% lower risk.

What could explain the link? Ascherio cannot say for certain."We know for certain caffeine gets into the brain. It activates several neurotransmitters related to depression." Among them, he says, are dopamine and serotonin.

That happens short-term, he says. How that affects the risk of depression long-term is not known, he says.

The new findings are consistent with some earlier research. He says one study found suicide risk is lower among those who drink more coffee. A previous study by others found that men who drink coffee have lower risks of depression.

Second Opinion

Although the study was well conducted, it has built-in limitations because it is observational, says Riba, past president of the American Psychiatric Society. The researchers simply looked at behaviors and then evaluated any associations.

"They are not saying cause and effect," she says.

With a group as large as the 50,000 studied by Ascherio, she says, the associations found can often disappear when the researchers look more closely.

"If we could prevent 20% of people from having depression, that would be terrific," Riba tells WebMD. "We have to continue to watch and follow this [research]."

Meanwhile, she cautions those with mood disorders to seek medical attention.

In an editor's note, Seth A. Berkowitz, MD, of the journal, writes: "This study makes an important contribution because it is, to my knowledge, the first large-scale study of coffee consumption to evaluate a mental health outcome in women."

He notes that other studies have found modest effects of caffeine on inflammation, no overall effect on heart disease, and perhaps modest protection against some cancers.

However, he adds that ''it seems premature to recommend coffee consumption" until research that looks at cause and effect is done.

The study was funded by the National Institutes of Health and the National Alliance for Research on Schizophrenia and Depression.